The glycosidic bond of β-galactosides can be cleaved by different classes of enzymes: (i) phospho-β-galactosidases (EC3.2.1.85) are specific for a phosphorylated substrate generated via phosphoenolpyruvate phosphotransferase system (PTS)-dependent uptake; (ii) typical β-galactosidases (EC3.2.1.23), represented by the Escherichia coli LacZ enzyme, which are relatively specific for β-galactosides; and (iii) β-glucosidases (EC 3.2.1.21) such as the enzymes of Agrobacterium faecalis, Clostridium thermocellum, Pyrococcus furiosus or Sulfolobus solfataricus (Day, A. G. and Withers, S. G., (1986) Purification and characterization of a β-glucosidase from Alcaligenes faecalis. Can. J. Biochem. Cell. Biol. 64, 914-922; Kengen, S. W. M., et al. (1993) Eur. J. Biochem., 213, 305-312; Ait, N., Cruezet, N. and Cattaneo, J. (1982) Properties of β-glucosidase purified from Clostridium thermocellum. J. Gen. Microbiol. 128, 569-577; Grogan, D. W. (1991) Evidence that β-galactosidase of Sulfolobus solfataricus is only one of several activities of a thermostable β-glycodiase. Appl. Environ. Microbiol. 57, 1644-1649). Members of the latter group, although highly specific with respect to the β-anomeric configuration of the glycosidic linkage, often display a rather relaxed substrate specificity and hydrolyse β-glucosides as well as β-fucosides and β-galactosides.
Generally, α-galactosidases are enzymes that catalyze the hydrolysis of galactose groups on a polysaccharide backbone or hydrolyze the cleavage of di- or oligosaccharides comprising galactose.
Generally, β-mannanases are enzymes that catalyze the hydrolysis of mannose groups internally on a polysaccharide backbone or hydrolyze the cleavage of di- or oligosaccharides comprising mannose groups. β-mannosidases hydrolyze non-reducing, terminal mannose residues on a mannose-containing polysaccharide and the cleavage of di- or oligosaccaharides comprising mannose groups.
Guar gum is a branched galactomannan polysaccharide composed of β-1,4 linked mannose backbone with a-1,6 linked galactose sidechains. The enzymes required for the degradation of guar are β-mannanase, β-mannosidase and α-galactosidase. β-mannanase hydrolyses the mannose backbone internally and β-mannosidase hydrolyses non-reducing, terminal mannose residues. α-galactosidase hydrolyses α-linked galactose groups.
Galactomannan polysaccharides and the enzymes that degrade them have a variety of applications. Guar is commonly used as a thickening agent in food and is utilized in hydraulic fracturing in oil and gas recovery. Consequently, galactomannanases are industrially relevant for the degradation and modification of guar. Furthermore, a need exists for thermostable galactomannases that are active in extreme conditions associated with drilling and well stimulation.
There are other applications for these enzymes in various industries, such as in the beet sugar industry. 20-30% of the domestic U.S. sucrose consumption is sucrose from sugar beets. Raw beet sugar can contain a small amount of raffinose when the sugar beets are stored before processing and rotting begins to set in. Raffinose inhibits the crystallization of sucrose and also constitutes a hidden quantity of sucrose. Thus, there is merit to eliminating raffinose from raw beet sugar. α-Galactosidase has also been used as a digestive aid to break down raffinose, stachyose, and verbascose in such foods as beans and other gassy foods.
β-Galactosidases which are active and stable at high temperatures appear to be superior enzymes for the production of lactose-free dietary milk products (Chaplin, M. F. and Bucke, C. (1990) In: Enzyme Technology, pp. 159-160, Cambridge University Press, Cambridge, UK). Also, several studies have demonstrated the applicability of β-galactosidases to the enzymatic synthesis of oligosaccharides via transglycosylation reactions (Nilsson, K. G. I. (1998) Enzymatic synthesis of oligosaccharides. Trends Biotechnol. 6, 156-264; Cote, G. L. and Tao, B. Y. (1990) Oligosaccharide synthesis by enzymatic transglycosylation. Glycoconjugate J. 7, 145-162). Despite the commercial potential, only a few β-galactosidases of thermophiles have been characterized so far. Two genes reported are β-galactoside-cleaving enzymes of the hyperthermophilic bacterium Thermotoga maritima, one of the most themiophilic organotrophic eubacteria described to date (Huber, R., Langworthy, T. A., König, H., Thomm, M., Woese, C. R., Sleytr, U. B. and Stetter, K. O. (1986) T. martima sp. nov. represents a new genus of unique extremely thermophilic eubacteria growing up to 90° C., Arch. Microbiol. 144, 324-333) one of the most thermophilic organotrophic eubacteria described to date. The gene products have been identified as a β-galactosidase and a β-glucosidase.
Pullulanase is well known as a debranching enzyme of pullulan and starch. The enzyme hydrolyzes α-1,6-glucosidic linkages on these polymers. Starch degradation for the production or sweeteners (glucose or maltose) is a very important industrial application of this enzyme. The degradation of starch is developed in two stages. The first stage involves the liquefaction of the substrate with α-amylase, and the second stage, or saccharification stage, is performed by β-amylase with pullalanase added as a debranching enzyme, to obtain better yields.
Endoglucanases can be used in a variety of industrial applications. For instance, the endoglucanases of the present invention can hydrolyze the internal β-1,4-glycosidic bonds in cellulose, which may be used for the conversion of plant biomass into fuels and chemicals. Endoglucanases also have applications in detergent formulations, the textile industry, in animal feed, in waste treatment, and in the fruit juice and brewing industry for the clarification and extraction of juices.
The polynucleotides and polypeptides of the present invention have been identified as glucosidases, α-galactosidases, β-galactosidases, β-mannosidases, β-mannanases, endoglucanases, and pullalanases as a result of their enzymatic activity.
The publications discussed herein are provided solely for their disclosure prior to the filing date of the present application. Nothing herein is to be construed as an admission that the invention is not entitled to antedate such disclosure by virtue of prior invention.